1. Field of the Invention
The invention relates to compact circuit breakers; and more particularly to such circuit breakers intended for DC circuits operating at relatively low voltages.
Most circuit breakers are rated only for AC use; at the usual U.S. power line frequency of 60 Hz, even if complex structures enabling very fast current limiting are not provided, the reduction of current to zero twice each cycle can still be relied on to keep event time (arc duration) to a maximum of about 8 milliseconds. However, when short circuit DC currents must be interrupted, event times as long as 18 to 30 msec may cause severe damage to internal parts because of overheating.
2. Description of the Prior Art
U.S. Pat. No. 4,970,481 discloses a relatively compact circuit breaker intended for AC use. Because of the proximity of the U-bend end of the fixed contact arm to the movable contact arm when the breaker has opened, there may be a tendency for one end of the arc to transfer from the fixed contact to the arm, rather than to move into the arc chute and be quickly extinguished. To prevent this transfer, a plastic arc shield is fastened over the fixed contact arm adjacent the contact, on the side of the contact away from the arc chute. The arc shield is described only as being formed of plastic and as being insulative.
The '481 patent also refers to U.S. Pat. No. 4,733,032 as describing an arc chute configuration, and composition of the chute support material, which are optimized to rapidly cool and extinguish the arc; and in particular refers to the arc quenching properties of the specific plastic resin material used. The '032 patent teaches a two layer construction of the chute side plates. The outer layer has woven glass fibers, and the inner layer has woven linen fibers, both layers being impregnated with a melamine/formaldehyde resin selected to evolve substantial quantities of high ionization potential gaseous material when heated by the arc.